The invention relates to a compressor arrangement for compressing a fluid, such as natural gas.
Motor vehicles, operating with compressed natural gas as an engine fuel, require the gas to be compressed to around 200 bar in order to store sufficient quantity in a volume comparable with liquid fuel. Conventionally reciprocating gas compressors have been used of the type using rotary movement to reciprocate the piston. Such reciprocating gas compressors usually operate with a number of stages in sequence such that the compression ratio in each stage is between 3:1 and 7:1. The operating speed of the piston in this type of compressor may be around 10 Hz and intercooling is provided between each compression stage to dissipate the heat generated when the gas is compressed. In these relatively high speed compressors, designs to achieve gas tight sealing are expensive particularly at pressures up to 200 bar.
The invention is concerned with providing a reduced cost arrangement with other advantages over the known arrangements.
According to the invention there is provided a fluid compressor having at least one stage of compression including two chambers each for receiving a first fluid to be compressed and means for receiving a source of second fluid under pressure to effect compression of the first fluid by reducing the volume within the chamber.
Preferably the compressor includes partition means in each chamber for separating the first and second fluids and switching means are provided to allow the source of pressurised fluid to alternate between each chamber to compress the first and second chambers alternatively by operating on the partition means.
Further according to the invention there is provided a method of compressing a fluid comprising the steps of providing the fluid to be compressed to a first or second fluid chamber, providing a source of pressurised second fluid to the first or second chamber to reduce the volume within the respective chamber to compress the other fluid.
Preferably the method includes the steps of: allowing the first chamber to open to receive the first fluid; thereafter reducing the size of the chamber to compress the fluid by means of the second pressurised fluid, and at the same time allowing the first fluid into the second chamber; and thereafter reducing the volume of the second chamber to compress, the fluid by means of the second pressurised fluid, and at the same time allowing the first fluid into the first chamber.
Hence in order to reduce the manufacturing cost and maintenance requirement for compressing relatively small volumes of gas, a slow moving hydraulically operated piston type compressor device is proposed. This utilises the ability of compact hydraulic pumps to deliver significant energy with a low volume flowrate of fluid at a pressure similar to the final gas pressure required (200 bar). In the proposed design, the speed of operation of the pistons is around 10 cycles/min rather than 10 cycles/sec (i.e. 60 times slower) thus reducing the wear rate on seals and allowing time for heat to dissipate. A higher speed version, with additional liquid cooling, for mounting on the vehicle could be employed but still of significantly lower speed. A further advantage of these designs is that the piston seals have more uniform pressures across them with the gas pressure being balanced by a similar or even higher hydraulic fluid pressure eliminating gas leakage across the seals.
High gas compression ratios, up to 250:1, can be achieved in a single stage compressor. Alternatively, a two stage version, with up to 15:1 compression ratio in each stage is possible with the added advantage of lower hydraulic oil flow rate and less peak power requirement, than in a single stage version, typically 1 L/min of oil flow for every 8 L/min of swept gas volume.